Glow plugs are well, known in the prior art and it is also well known by those skilled in the art that an electrical resistance or resistances are generally embedded in an electrically insulating powder, so as to be electrically insulated from the tubular sheath they are located in, except for electrical connection with the free closed end of said tubular sheath. It is further known in prior art that in glow plugs using one single electrical resistance, the resistance may have positive temperature coefficient characteristics (PTC characteristics) and that in glow plugs using two in series connected electrical resistances the resistance which is connected to the electrode of the glow plug has higher PTC characteristics than the resistance which is connected to the free closed end of the tubular sheath.
Prior art glow plugs of the above described type using one single electrical resistance are for example disclosed in U.S. Pat. Nos. 4,477,717 and 4,639,712 whilst prior art glow plugs using two electrical resistances are for example disclosed in U.S. Pat. Nos. 4,423,309 and 4,582,980. In these four patents, and generally in the prior art, the electrical resistance or resistances located in the tubular sheath are totally embedded in one single electrically insulating powder, even if the powder may be composed of a mixture of several different materials. The powder not only has good electrically insulating characteristics, but also has good thermal conductivity characteristics. The good thermal conductivity characteristics of the powder used in prior art are necessary for the rapid transfer of the heat produced by the electrical resistance or resistances to the exterior of the tubular sheath of the glow plug. However, the use of one single powder does not permit the optimization of one of the main qualities a glow plug for modern engines must have, i.e. a rapid increase of the temperature of the free closed end portion of the tubular sheath of the glow plug.